Stenosis
Coronary stenosis is the narrowing of coronary arteries by obstructive atherosclerotic plaques. The coronary arteries supply oxygenated blood flow to the myocardium. Although mild and moderate coronary stenosis do not impede resting coronary flow, stenosis >30-45% starts to restrict maximal coronary flow. Severe coronary stenosis (>70% reduction in luminal diameter) causes stable angina (ischemic chest pain upon exertion). Significant stenosis contributes, along with plaque rupture and thrombus formation, coronary spasm, or inflammation/infection, to unstable angina as well as myocardial infarction. Together with arrhythmia, coronary stenosis is a major factor of sudden cardiac deaths, as evidenced by its presence in two or more major coronary arteries in 90% of adult sudden cardiac death victims.
Coronary stenosis is a prevalent disease. Each year in the United States, 440,000 new cases of stable angina and 150,000 new cases of unstable angina occur. This year, an estimated 1.1 million Americans will have a new or recurrent heart attack. These incidences result in over six million individuals in the U.S. living with stable or unstable angina pectoris, a debilitating condition, and over seven million individuals in the U.S. living with a history of myocardial infarction. Coronary stenosis is frequently a deadly disease. It is a major underlying cause of coronary heart disease (CHD), which is the single largest cause of death in the U.S. Over half a million coronary deaths, including 250,000 sudden cardiac deaths, occur each year in U.S. Coronary stenosis is also a costly disease. It is the major reason for 1.2 million cardiac catheterizations, 0.4 million angioplasties, and 0.6 million bypass surgeries, contributing to the estimated 110 billion dollar total costs of CHD in the U.S. for the year 2002. Still, these statistics underestimate the true prevalence of the disease since coronary stenosis often remains clinically asymptomatic for decades, and only becomes symptomatic when the disease has progressed to a severe, and sometime fatal, state.
There is therefore an unmet need in early diagnosis and prognosis of asymptomatic coronary stenosis. This need is particularly significant given that early diagnosis or prognosis results can significantly influence the course of disease by influencing treatment choices (for example, those with genetic risks can be treated to modify risk factors such as hypertension, diabetes, inactivity, dyslipidemia, etc.), thresholds (e.g., lipid levels used to trigger the use of lipid-lowering drugs), and goals (e.g., target blood pressure or lipid levels), and possibly enhance compliance.
Diagnosis of coronary stenosis currently starts by assessing if the risk profiles (e.g., hypertension, dyslipidemia, family history, diabetes, etc.) and symptoms (e.g., angina) of patients are consistent with coronary heart disease, followed most commonly by resting and exercise EKGs. However, risk assessments and EKGs are imperfect diagnostic tests for stenosis since they can be both insensitive (giving false negatives) and non-specific (giving false positives). Coronary arteriography is the definitive test for assessing the severity of coronary stenosis, however, it is not very sensitive in early detection of mild stenosis. It is also an invasive procedure with a small risk of death due to the catheterization procedure and the contrast dye. Because of this risk, it is typically only used at a time when coronary stenosis is considered likely from symptoms or other tests, which is hardly an ideal time to start intervention.
Coronary stenosis risk is presumed to have a strong genetic component. It is well known that several major risk factors of coronary disease are heritable, e.g. serum lipid levels (Perusse L. et. al., Arterioscler Thromb Vasc Biol (1997): 17(11) 3263-9) and obesity (Rice T. et. al., Int J Obes Relat Metab Disord (1997):21(11) 1024-31). Indeed, several known genetic defects are individually sufficient to cause elevated serum LDL-cholesterol (e.g., familial hypercholesterolemia) leading to premature coronary disease (Goldstein and Brown, Science 292 (2001): 1310-12). In addition, linkage studies in humans have replicated the findings of the link of several chromosomal regions (quantitative trait loci) to coronary heart disease and related diseases and risk factors (Pajukanta P. et. al., Am J Hum Genet 67 (2000):1481-93, Francke S. et. al., Human Molecular Genetics (2001): 10 (24) 2751-65). Finally, a family history of premature coronary disease is a significant factor in the risk assessment and diagnosis of coronary disease (Braunwald E., Zipes D. and Libby P., Heart Disease, 6th ed. W. B. Saunders Company, 2001, 28).
Although many risk factors for coronary stenosis have been identified, including age, diabetes, hypertension, high serum cholesterol, smoking, etc., and genetic factors play significant roles in several of these risk factors, significant genetic risk factors are likely to exist which have not been identified to date. In addition to the anecdotal coronary disease patients that exhibit few traditional risk factors, a study of multiple existing risk factors showed that only half of the “population-attributable risk” was attributable to known risk factors (Change M. et. al., J Clin Epidemiol (2001) 54 (6) 634-44). Therefore, the presently known risk factors are inadequate for predicting coronary stenosis risk in individuals. Given the magnitude of the disease, there is an urgent need for genetic markers that are predictive of coronary stenosis risk. Such genetic markers could increase the prognostic ability of existing risk assessment methods and complement current diagnostic methods such as exercise EKG, especially in early detection of disease when intervention is most effective and should ideally start.
SNPs
The genomes of all organisms undergo spontaneous mutation in the course of their continuing evolution, generating variant forms of progenitor genetic sequences (Gusella, Ann. Rev. Biochem. 55, 831-854 (1986)). A variant form may confer an evolutionary advantage or disadvantage relative to a progenitor form or may be neutral. In some instances, a variant form confers an evolutionary advantage to the species and is eventually incorporated into the DNA of many or most members of the species and effectively becomes the progenitor form. Additionally, the effects of a variant form may be both beneficial and detrimental, depending on the circumstances. For example, a heterozygous sickle cell mutation confers resistance to malaria, but a homozygous sickle cell mutation is usually lethal. In many cases, both progenitor and variant forms survive and co-exist in a species population. The coexistence of multiple forms of a genetic sequence gives rise to genetic polymorphisms, including SNPs.
Approximately 90% of all polymorphisms in the human genome are SNPs. SNPs are single base positions in DNA at which different alleles, or alternative nucleotides, exist in a population. The SNP position (interchangeably referred to herein as SNP, SNP site, or SNP locus) is usually preceded by and followed by highly conserved sequences of the allele (e.g., sequences that vary in less than 1/100 or 1/1000 members of the populations). An individual may be homozygous or heterozygous for an allele at each SNP position. A SNP can, in some instances, be referred to as a “cSNP” to denote that the nucleotide sequence containing the SNP is an amino acid coding sequence.
A SNP may arise from a substitution of one nucleotide for another at the polymorphic site. Substitutions can be transitions or transversions. A transition is the replacement of one purine nucleotide by another purine nucleotide, or one pyrimidine by another pyrimidine. A transversion is the replacement of a purine by a pyrimidine, or vice versa. A SNP may also be a single base insertion or deletion variant referred to as an “indel” (Weber et al., “Human diallelic insertion/deletion polymorphisms”, Am J Hum Genet 2002 October; 71(4):854-62).
A synonymous codon change, or silent mutation/SNP (terms such as “SNP”, “polymorphism”, “mutation”, “mutant”, “variation”, and “variant” are used herein interchangeably), is one that does not result in a change of amino acid due to the degeneracy of the genetic code. A substitution that changes a codon coding for one amino acid to a codon coding for a different amino acid (i.e., a non-synonymous codon change) is referred to as a missense mutation. A nonsense mutation results in a type of non-synonymous codon change in which a stop codon is formed, thereby leading to premature termination of a polypeptide chain and a truncated protein. A read-through mutation is another type of non-synonymous codon change that causes the destruction of a stop codon, thereby resulting in an extended polypeptide product. While SNPs can be bi-, tri-, or tetra-allelic, the vast majority of the SNPs are bi-allelic, and are thus often referred to as “bi-allelic markers”, or “di-allelic markers”.
As used herein, references to SNPs and SNP genotypes include individual SNPs and/or haplotypes, which are groups of SNPs that are generally inherited together. Haplotypes can have stronger correlations with diseases or other phenotypic effects compared with individual SNPs, and therefore may provide increased diagnostic accuracy in some cases (Stephens et al. Science 293, 489-493, 20 July 2001).
Causative SNPs are those SNPs that produce alterations in gene expression or in the expression, structure, and/or function of a gene product, and therefore are most predictive of a possible clinical phenotype. One such class includes SNPs falling within regions of genes encoding a polypeptide product, i.e. cSNPs. These SNPs may result in an alteration of the amino acid sequence of the polypeptide product (i.e., non-synonymous codon changes) and give rise to the expression of a defective or other variant protein. Furthermore, in the case of nonsense mutations, a SNP may lead to premature termination of a polypeptide product. Such variant products can result in a pathological condition, e.g., genetic disease. Examples of genes in which a SNP within a coding sequence causes a genetic disease include sickle cell anemia and cystic fibrosis.
Causative SNPs do not necessarily have to occur in coding regions; causative SNPs can occur in, for example, any genetic region that can ultimately affect the expression, structure, and/or activity of the protein encoded by a nucleic acid. Such genetic regions include, for example, those involved in transcription, such as SNPs in transcription factor binding domains, SNPs in promoter regions, in areas involved in transcript processing, such as SNPs at intron-exon boundaries that may cause defective splicing, or SNPs in mRNA processing signal sequences such as polyadenylation signal regions. Some SNPs that are not causative SNPs nevertheless are in close association with, and therefore segregate with, a disease-causing sequence. In this situation, the presence of a SNP correlates with the presence of, or predisposition to, or an increased risk in developing the disease. These SNPs, although not causative, are nonetheless also useful for diagnostics, disease predisposition screening, and other uses.
An association study of a SNP and a specific disorder involves determining the presence or frequency of the SNP allele in biological samples from individuals with the disorder of interest, such as stenosis, and comparing the information to that of controls (i.e., individuals who do not have the disorder; controls may be also referred to as “healthy” or “normal” individuals) who are preferably of similar age and race. The appropriate selection of patients and controls is important to the success of SNP association studies. Therefore, a pool of individuals with well-characterized phenotypes is extremely desirable.
A SNP may be screened in diseased tissue samples or any biological sample obtained from a diseased individual, and compared to control samples, and selected for its increased (or decreased) occurrence in a specific pathological condition, such as pathologies related to stenosis. Once a statistically significant association is established between one or more SNP(s) and a pathological condition (or other phenotype) of interest, then the region around the SNP can optionally be thoroughly screened to identify the causative genetic locus/sequence(s) (e.g., causative SNP/mutation, gene, regulatory, region, etc.) that influences the pathological condition or phenotype. Association studies may be conducted within the general population and are not limited to studies performed on related individuals in affected families (linkage studies).
Clinical trials have shown that patient response to treatment with pharmaceuticals is often heterogeneous. There is a continuing need to improve pharmaceutical agent design and therapy. In that regard, SNPs can be used to identify patients most suited to therapy with particular pharmaceutical agents (this is often termed “pharmacogenomics”). Similarly, SNPs can be used to exclude patients from certain treatment due to the patient's increased likelihood of developing toxic side effects or their likelihood of not responding to the treatment. Pharmacogenomics can also be used in pharmaceutical research to assist the drug development and selection process. (Linder et al. (1997), Clinical Chemistry, 43, 254; Marshall (1997), Nature Biotechnology, 15, 1249; International Patent Application WO 97/40462, Spectra Biomedical; and Schafer et al. (1998), Nature Biotechnology, 16, 3).